1. Field of the Invention
The present invention relates to a camera for taking a picture in a good focus condition.
2. Description of the Related Art
In recent years, technology regarding a photographic film is improved so that the photographic film is adapted to have high sensitivity without deteriorating image quality. The photographic films of ISO 1000 and ISO 1600, for instance, tend to be used in general. When this kind of photographic film having high sensitivity is used in a room and at the midnight outdoors, it is possible to take a picture in proper brightness, whereas the background thereof becomes dark on another photographic film having comparatively low sensitivity. For example, when a picture is taken on condition that f-number of an exposure aperture of a camera is F4, proper exposure is obtained relative to subject brightness of BV0 by combining with a shutter speed of {fraction (1/30)} sec and by using the photographic film having film sensitivity of ISO 1600. Thus, an exposure amount is properly obtained relative to the background in a photographic scene of general room lighting and in a photographic scene having the background of a bright night view.
Most of the currently-sold cameras comprise an automatic exposure mechanism (AE mechanism) and an autofocus mechanism. In a compact camera comprising the AE mechanism, the f-number and the shutter speed are determined so as to correspond to a proper exposure value, which is determined on the basis of measured subject brightness and sensitivity of the loaded photographic film. Many compact cameras use a program shutter controlling the exposure amount in accordance with the determined f-number and shutter speed.
Although there are various programs used for the program shutter, many compact cameras adopt a program shifting the shutter speed to a high-speed side. FIG. 12 shows an example of this kind of program. Such as shown in FIG. 12, as the exposure value (EV) becomes large, the shutter speed is gradually shifted to the high-speed side, keeping the f-number of the aperture (F4 in FIG. 12). When the shutter speed becomes high ({fraction (1/250)} sec in FIG. 12), the shutter speed is shifted to the high-speed side, increasing the f-number to close down a stop. The reason for adopting such a program is that camera-shake is prevented from occurring when using a telephoto lens or the like.
The camera including the autofocus mechanism is provided with a measurement area, which is normally arranged at a central portion of a picture to be taken, to measure a distance to a subject existing in the measurement area. A projection amount of a taking lens is adjusted so as to automatically bring the measured distance into focus.
By the way, when using the high-sensitivity photographic film having the film sensitivity of ISO 1000 and ISO 1600, the exposure is performed within an exposure control range of the general compact camera so that it is possible to obtain a proper exposure amount. In a case of the program shutter shown in FIG. 12, the proper exposure amount is about EV9 to EV13 upon using the photographic film of ISO 1600 when a picture is taken inside a room of which subject brightness is about BV0 to BV4. In this case, although the proper exposure amount is obtained, a depth of field becomes shallow, since the f-number of the aperture is used. Accordingly, there arises a problem in that all of the taken pictures have great blur with respect to the background, whereas the main subject is brought into focus.
Further, when the autofocus mechanism of the camera is used for taking a picture, photographing is mistakenly performed if the measurement area is not set to the main subject at the time of framing. In this case, the main subject is not brought into focus, but the background is brought into focus.
In view of the foregoing, it is a primary object of the present invention to provide a camera in which it is possible to take a picture in a good focus condition.
It is a second object of the present invention to provide a camera in which a main subject of a short distance and the background thereof are photographed in a good focus condition even if photographing is performed in a room.
In order to achieve the above and other objects, the camera according to the present invention comprises a program shutter and a focal-position controller limiting a length for focusing within a range of 3 m or less. In the program shutter, a combination of a shutter speed and an f-number corresponding to each exposure value is determined so as to satisfying the following condition:
0.5xe2x89xa6(AV11xe2x88x92AV9)/(TV11xe2x88x92TV9)xe2x89xa62.0xe2x80x83xe2x80x83(1) 
wherein AV9 denotes an apex value of the f-number which is set when the exposure value is EV9, and TV9 denotes an apex value of the shutter speed at that time. Further, AV11 denotes the apex value of the f-number which is set when the exposure value is EV11, and TV11 denotes the apex value of the shutter speed at that time.
In another embodiment, the camera according to the present invention comprises a program shutter and a taking lens of a fixed-focus type in which a length for focusing is set within a range of 1.0 m to 3.0 m. In the program shutter, the combination of the shutter speed and the f-number corresponding to each exposure value is determined so as to satisfying the foregoing condition (1).
In the present invention, a program of the program shutter is adapted to satisfy the forgoing condition (1). At the same time, when film sensitivity is ISO 1000 or more, the length for focusing is limited within a range of 3 m or less. Owing to this, when a photographic film of ISO 1000 or more is used, a main subject of a short distance and the background thereof can be photographed in a good focus condition even in a photographic environment of a room and so forth. Further, in the case that the taking lens of the fixed-focus type is used, the length for focusing is set within the range of 1.0 m to 3.0 m so that a main subject of a short distance and the background thereof can be photographed in a good focus condition even in a photographic environment of a room and so forth.